Optical communication systems are becoming increasingly important for data transmission because they offer high transmission speeds and high bandwidth. The success of optical communication systems depends critically on the quality of optical fibers used in data transmission systems. Optical fibers must transfer optical data signals with high fidelity and low attenuation.
Optical fibers are made by drawing fibers from a preform. The preform is consolidated silica glass that typically includes a series of concentric regions of silica glass that differ in the level or type of dopant. Control of the spatial distribution, concentration, and/or type of dopant in the fiber preform creates regions that differ in refractive index. The differences in refractive index are manifest in fibers drawn from the preform and define the different functional regions of an optical fiber (e.g. core vs. cladding, low index depressions, tailored index profiles).
One conventional process for making optical fiber preforms is an outside vapor deposition process that entails deposition of silica (or doped silica) soot onto a silica (or doped silica) cane. The cane is fully consolidated glass with a generally cylindrical geometry and becomes the central portion of the fiber preform. The cane has the composition desired for the high index core region of the fiber ultimately drawn from the preform (and for this reason is often referred to as the core cane). The silica soot surrounds the cane and can be deposited as a single layer with a single composition or a series of layers that differ in composition, where the compositions of the one or more layers are designed to provide the index profile desired in the cladding region of the fiber ultimately drawn from the preform. The one or more soot cladding layers typically include undoped silica and doped silica layers that differ in concentration or type of dopant.
Cladding soot is usually produced by flame reaction of one or more precursors. The flame reaction may be flame hydrolysis or flame combustion. In flame hydrolysis, water is present as a reactant and reacts with a soot precursor to form cladding soot. In flame combustion, water is not a reactant, but may be produced as a byproduct. Common precursors for silica soot include SiCl4 and OMCTS (octamethylcyclotetrasiloxane). The presence of water in the soot deposition reaction can lead to high concentrations of OH in the silica soot and at the surface and near-surface region of the cane. To reduce the concentration of OH groups, a dehydration step is performed after soot deposition. In the dehydration step, the soot and cane are exposed to a dehydration agent (e.g. Cl2) that acts to remove OH. The high porosity of the as-deposited soot facilitates removal of OH from the soot layer in the dehydration step. The densified nature of the cane, however, inhibits penetration of the cane by the dehydration agent and significant amounts of OH can remain in the cane portion of the preform. The presence of OH in the preform leads to incorporation of a high concentration of OH in fibers drawn from the preform and to undesirably high fiber attenuation losses for optical signals at or near 1380 nm due to a broad OH absorption band that extends from ˜1350 nm to 1425 nm.
Since the optical signal in a transmission fiber is confined primarily to the core region, it is especially important to minimize the OH concentration in the fiber core, which requires minimization of the OH concentration in the core cane (the region of the fiber preform from which the fiber core is drawn). The typical strategy used to minimize the presence of OH in the core cane is to localize the high index region toward the center of the core cane. The high index region of the core cane is typically formed from updoped silica (e.g. Ge-doped silica) and the region of updoping is limited to a central portion of the core cane. The objective is to maintain the updoped region at a sufficient distance from the outer radial boundary of the core cane to protect the high index region from OH contamination. The portion of the core cane between the central updoped region and outer radial boundary acts as a buffer to inhibit diffusion of OH formed on the surface of the core cane. Because of the consolidated state of the core cane, diffusion of OH from the surface to the center of the core cane does not occur on practical time scales and OH is localized at the surface and near surface regions of the core cane. By positioning the high index region in the interior of the core cane at a sufficient distance from the near surface region, the presence of OH in the high index region can be minimized and attenuation losses due to OH absorption are avoided.
The high index region is typically centered in the core cane and the radial extent of the high index region can be quantified by the core-clad ratio of the core cane. The core-clad ratio is defined as the ratio of the radius of the high index region to the outer radius of the core cane. A core-clad ratio of 0.5, for example, signifies that the radius of the high index (updoped) region of the core cane is half the total radius of the core cane. In the conventional outside vapor deposition process, the core-clad ratio is kept small (e.g. <0.33) to minimize the presence of OH in the updoped region of the preform and in the core of fibers drawn from the preform. Utilization of core canes with a low core-clad ratio, however, is not economical from a process perspective because of the time and material costs required to enlarge the core cane beyond the dimensions of the updoped region.
The cane-in-soot process is an alternative method for making fiber preforms that avoids exposure of the core cane to water. In the cane-in-soot process, a core cane and soot cladding monolith are formed in separate processes and subsequently joined to form a core-cladding assembly that is consolidated to form a preform. The soot cladding monolith is porous and includes an internal cavity in which the core cane is placed. Consolidation densifies the porous soot cladding monolith and fuses the core cane to the soot cladding monolith to form an integral body that can be used as a fiber preform. Because the core cane and soot cladding monolith are formed independently, the core cane is not exposed to water reactants or byproducts present in the cladding soot deposition process. The core cane can be formed, dehydrated and consolidated in an environment free of water. Similarly, the soot cladding monolith can be deposited and dehydrated while in a porous state to essentially eliminate OH before joining of the core cane with the soot cladding monolith. Insertion of the core cane into the internal cavity of the porous soot cladding monolith occurs in the absence of water. Concerns over incorporation of OH into the high index region of the core cane are thus alleviated and fibers with low attenuation can be produced.
Because the core cane is protected from water, the cane-in-soot process improves process efficiency by enabling the use of core canes having large core-clad ratios. Practical implementation of the cane-in-soot process, however, reveals the formation of defects in preforms made in the cane-in-soot process from core canes having large core-clad ratios. The defects are believed to originate from stresses that develop during cooling of the preform after consolidation in the cane-in-soot process. It would be desirable to develop a cane-in-soot process that permits formation of fiber preforms without defects from core canes having a large core-clad ratio.